We will investigate in the salamander the implications of the morphological association between mechanosensory nerve terminals and the epidermal Merkel cell that we have shown is the target site for ingrowing nerves. This association has a novel ultrastructural feature suggesting that a two-way transfer of information occurs across the junction between the nerve terminal and the Merkel cell. We are interested in the possibilities that this apparent reciprocal complex may be the morphological basis of: (1) two-way interactions of a trophic nature between nerve and target, and (2) two-way synaptic interactions that are involved in the rapidly-adapting characteristics of the mechanosensory response. Our recent studies suggest that a reduction in neuronal transport disturbs the nerve-Merkel cell trophic interactions, resulting in sprouting of other axons, apparently towards the involved Merkel cell; we have evidence that sprouting of this kind continues until the disturbance is offset through development of new contacts between neurites and Merkel cells. To understand better the nature of the nerve-Merkel cell trophic interactions that seem to regulate collateral sprouting, we are developing a relatively simple "in vitro" system in which isolated DRG neurons and skin cells are co-cultured. The Merkel cell-neurite junction also shows ultrastructural specializations expected of a reciprocal synapse. We have designed experiments, both "in vivo" and "in vitro" to test whether the rapidly adapting nature of the mechanosensory response is related to the ultrastructure of the Merkel cell-neurite complex, and whether ultrastructural changes in this complex are correlated with changes in physiological function. These studies will involve correlative electrophysiological and electronmicroscopical procedures.